Nutrient Physiology Of Betula Ermanii And Its Implications To Treeline Formation Of Changbai Mountains

Alpine treeline is one of the most conspicuous vegetation boundaries,the high altitude limit of montane forests distrubition.Treeline is highly sensitive to global and regional climate change and is considered as an ideal indicator of such changes.Betula ermanii treeline ecotone on Changbai Mountain is an ideal zone to study the effect of climate change on temperate alpine treeline because of its obvious regional characteristics to global change.Study on nutrient physiological characteristics of B.ermanii treeline on Changbai Mt.can reveal the response of regional ecology to climate change,becoming an important research in global climate change.In order to better understand the ecophysiological mechanisms of treeline formation and predict the change of alpine treeline in global climate change,especially under global climate warming,we used natural elevational gradients as proxy of changed temperature,which decreases with increasing elevation.On a cross-climate scale?northeast vs.southwest?,we investigated differences in resource remobilization between deciduous broad-leaved B.ermanii?northeast Changbai Mt.?and evergreen broad-leaved Quercus aquifolioides?southwest Balang Mountain?,and then investigated the response and adaption of resource remobilization eficiency to treeline habitats.To assess the nutrient use strategy of trees at treeline,we compared NSC,N,P and K concentrations,seasonal dynamics and remobilization efficiencies in each tissue along elevational gradients of six elevations from tree species line to lower edge of Erman's birch forest.The in situ¹³CO₂ pulse labeling was conducted on B.ermanii grown at low elevation?1700 m a.s.l.?and treeline elevation?2000 m a.s.l.?on Changbai Mt..We aimed to trace the fate of¹³C in plant-soil system and illustrate the photosynthetically fixed ¹³C and allocation in trees at treeline.?1?Mobile carbohydrates and nutrients?N and P?,in both the deciduous B.ermanii and evergreen species Q.aquifolioides,were remobilized from leaves to shoots at the end of season.Moreover,due to the different strategies of evergreen and deciduous species in use of nutrients,compared to the deciduous B.ermanii trees,the evergreen species Q.aquifolioides remobilized more carbon components,but less N and P from leaves to shoots.?2?B.ermanii trees reallocate resources?NSC,N,P and K?from leaves to storage tissues at the end of growing season.Our study indicated that stems and roots of deciduous trees at high elevations are the most important storage tissues for resources overwinter.We found trees had lower end-season reallocation rate of NSC,N,P,and K from leaves to woody tissues at the higher elevations above the alpine treeline compared to lower elevations.This lower reallocation rate may result in a resource limitation in high-elevation trees and further contribute to the alpine treeline formation.?3?The treeline B.ermanii trees had higher carbon assimilation rates than low-elevation trees.This result indicated that the photosynthesis of B.ermanii trees grown at alpine treeline were not influenced by low temperature.While the results for treeline trees indicated that whether the storage level or the concentration of newly produced soluble carbon was lower in sink tissues than low-elevation trees,although the differences were not significant.Therefore our results points toward a lower newly assimilated carbon of treeline trees allocated to sink tissues at the growing season,which possibly limit carbohydrates storage over-winter in treeline trees and thus result in insufficient carbon supply for next early spring.Our study summarized the mechanisms of treeline formation with a focus on resource production,distribution and transferation to storage.Our main results suggest that trees at cold treeline suffered from a winter resource storage shortage.Under global climate warming,increased temperature of treeline habitat will improve trees resource remobilization and storage at the end of season,and then promote the growth of trees at treeline,leading to the upward shifts of the alpine treeline.Therefore,the treeline of the Changbai Mt.would continue shifting upward to tundra under the future climate warming scenarios.